What is recently required of toners is a reduction in particle diameter from the viewpoint of improving the quality of output images, high-temperature anti-offset property, low-temperature fixability for energy saving, and heat-resistant storage property high enough to withstand high temperature and high humidity during storage or transport after the production thereof. In particular, since power consumption in toner fixation occupies a major part of power consumption in an image formation process, an improvement in low-temperature fixability is very important.
Toners prepared by a kneading/grinding method have hitherto been used. The toners prepared by the kneading/grinding method, however, involve difficulties in reducing the particle diameter and have irregular shapes and a broad particle size distribution, posing problems such as unsatisfactory output image quality and high fixation energy. Further, when wax is added (releasing agent) for fixability improvement purposes, toners prepared by the kneading/grinding method, when ground, are broken at the interface of wax and, thus, a large amount of the wax is disadvantageously present on the surface of the toners. Accordingly, a release effect is attained, but on the other hand, deposition (filming) of the wax on a carrier, a photoconductor, and a blade is likely to occur. Thus, the toners prepared by the kneading/grinding method are disadvantageously unsatisfactory in the overall properties.
Therefore, in order to overcome the problems involved in the kneading/grinding method, a method for producing toners by a polymerization method has been proposed. In the polymerization method, toners having a reduced particle diameter can easily be produced, and the toners have a sharper particle size distribution than the toners produced by the grinding method. Further, embedding of the releasing agent is also possible. The production of a toner from an extension reaction product of a urethane-modified polyester as a toner binder has been proposed as a process for producing a toner by the polymerization method with a view to improving the low-temperature fixability and improving the high-temperature anti-offset property (see, for example, PTL 1).
Further, a process for producing a toner that excels in powder flowability and transferability in a small particle diameter form and, at the same time, excels in all of the heat-resistant storage property, the low-temperature fixability, and the high-temperature anti-offset property has been proposed (see, for example, PTLs 2 and 3).
Further, a process for producing a toner including the production of a toner binder having a stable molecular-weight distribution and the step of aging to simultaneously satisfy the low-temperature fixability and the high-temperature anti-offset property has been proposed (see, for example, PTLs 4 and 5).
These proposed techniques, however, do not meet a high level of low-temperature fixability that has recently been required.
Accordingly, a toner that contains a resin including a crystalline polyester resin and wax (a releasing agent) which are incompatible with each other and have an islands-sea-type phase-separated structure has been proposed with a view to providing a high level of low-temperature fixability (see, for example, PTL 6).
Further, a toner containing a crystalline polyester resin, a release resin, and a graft polymer has been proposed (see, for example, PTL 7).
These proposed techniques are advantageous in that the crystalline polyester resin is melted earlier than the amorphous polyester resin and, thus, low-temperature fixation can be realized. However, even when the crystalline polyester resin constituting islands in the islands-sea-type phase-separated structure is melted, the amorphous polyester resin constituting sea that occupies a major part of the islands-sea structure remains unmelted. Fixation does not occur until both the crystalline polyester resin and the amorphous polyester resin are melted to some extent. Accordingly, these proposed techniques do not meet a high level of low-temperature fixability that has recently been required.
Further, in addition to excellent low-temperature fixability, high-temperature anti-offset property, and heat-resistant storage property, good color reproducibility is also required of toners because of a requirement for high-quality images.
Techniques in which a fluorescent brightening agent is incorporated in a toner have been proposed (see, for example, PTLs 8 and 9). In these techniques, however, the purpose of adding the fluorescent brightening agent is not to enhance the color reproducibility of visible images.
In general, organic pigments having excellent heat resistance and lightfastness are used in toners. When a resin having poor spreadability is used as the resin in the toner, in a secondary color of blue, red, or green prepared by superimposing two different color toners on top of each other, the color toner constituting the lowermost layer is disadvantageously concealed by the color toner constituting the overlying layer in the superimposed individual color toners. Accordingly, the color of the toner constituting the lowermost layer cannot be viewed without difficulties, and the chroma is lowered, resulting in a drawback that the color reproducibility of images is inhibited.
Therefore, when the high-temperature anti-offset property and the heat-resistant storage property are improved, the color reproducibility is disadvantageously lowered.
Accordingly, at the present time, toners that have excellent low-temperature fixability, high-temperature anti-offset property, and heat-resistant storage property and, at the same time, have excellent color reproducibility have been desired.